The present invention relates to additives for electrochemical cells which can eliminate or reduce undesirable effects which can arise during storage and abuse of such cells.
The history of electrochemical cells goes back to 1866 when Leclanchxc3xa9 first discovered the principle on which they are based. The manufacture and design of electrochemical cells has come a long way since that time, but problems still remain. Cells (also known as batteries, although the term technically relates to a series of cells) essentially consist of an anode, a cathode and an electrolyte. In the present day version of the Leclanchexc3xa9 cell, the anode is zinc, the cathode is manganese dioxide and the electrolyte is an aqueous solution of varying proportions of zinc chloride and ammonium chloride. In other primary cells, the electrolyte is frequently an aqueous solution of potassium or sodium hydroxide. In any event, it is necessary to seal the various components into a can in order to prevent the possibly dangerous escape of the constituents, as well as to prevent the atmosphere from affecting the constituents.
The problem of leakage of the electrolyte and corrosion of the can (zinc in Leclanche cells) was very largely overcome by the addition of cadmium and mercury, but especially mercury, to the cell ingredients.
Thus, mercury was responsible for reducing perforation of the can during abuse conditions, reducing corrosion and preventing perforation during storage, and it also had the advantage that it assisted in discharge. However, now that mercury is viewed as a major environmental pollutant, there has been a very major push to develop cells with no added mercury and, to a lesser extent, cells with no added cadmium.
The essential problem with cells which have no added mercury is that no one has yet found any additive which is capable of recreating the advantages of cells which contain mercury. In fact, even the optimum selection of all of the currently known additives is not as good as mercury.
Some of the known additives which have been looked at include, for example: the arylsulphur compounds of EP-A-421660 (which prevent leakage and perforation but do not control gassing); the fluoroalkylpolyoxyethylene ether compounds of U.S. Pat. No. 4,606,984 (which control gassing but which have no effect on corrosion, leakage or perforation); the alkyl polyoxyethylene ethers of U.S. Pat. No. 3,847,669 and the alkyl polyoxyethylene phosphate ethers of GB-A-2170946 (which control gassing but nothing else); and the tetraalkyl and alkyl ammonium compounds of U.S. Pat. No. 3,945,849 (which prevent corrosion, leakage and perforation but not gassing and which also have poor electrical performance).
In addition, known additives, such as listed above, while having certain beneficial effects, have the unfortunate side-effect of reducing performance. In order to assay this, cells are kept at high temperatures for long periods (for example, 13 weeks at 45xc2x0 C. and 50% r.h. [relative humidity]). Performance retention is then calculated as a measure of performance compared with similar batteries kept for two weeks at 20xc2x0 C., the results ideally being in excess of 80%. In fact, the above listed additives provide performance retention generally in the region of only 75% by comparison with cells containing no additives, such cells typically having a performance retention in the region of 82%.
Apart from the necessary anode, cathode and electrolyte, practical considerations demand that a separator is provided between the anode and the cathode in order to avoid possible contact between the anode and cathode, which could lead to undesirable short circuiting.
In general, one of two types of separator is employed, and is either a gel/paste composition or coated paper. With the drive to greater efficiency and performance, the coated paper separators are particularly preferred, as they take up less space in the cell.
The coated paper separators are coated with starch which, in the presence of the electrolyte, is tonically conductive, but not electronically conductive.
We have now, surprisingly, established that polyoxyalkylene nitrogen containing compounds can be used as additives in electrochemical cells in order to alleviate or even overcome many of the problems associated with cells having no added mercury.
Thus, in a first aspect, the present invention provides an additive for use in an electrochemical call having an acidic electrolyte, characterised in that the additive is a polyoxyalkylene nitrogen containing compound.
Additives of the present invention are useful to help inhibit gassing and leakage in electrochemical cells, especially cells with no added mercury, as well an having minimal adverse effect on the performance of the cell.
In fact, we have found that polyoxyalkylene nitrogen containing compounds generally perform as well as, or better than, any single other additive intended to prevent gassing, leakage or corrosion. In addition, we have also found that performance is often enhanced. Thus, the compounds of the invention are generally useful to reduce corrosion, gassing and leakage, a combination which has not previously been observed for any single additive.
Furthermore, as stated above, arylalkylpolyoxyethylene ether and trimethylalkyl ammonium both have good antigassing properties, but a combination of these two compounds does not have the same effect as the compounds of the present invention. Whilst the combination of compounds is slightly better than arylalkylpolyoxyethylene ether alone, the compounds of the present invention are generally capable of reducing gassing by up to half, or more, of the levels observed with the arylalkylpolyoxyethylene ether additives.
The nitrogen containing compounds of the present invention may be of any type that is suitable to be substituted by one or more polyoxyalkylene groups. Whilst amine and ammonium compounds are preferred, especially the amine compounds, other compounds which have substitutable nitrogen bonds are also suitable, such as carbamoyl, diazo and aci-nitro compounds.
The individual alkylene moieties in the polyoxyalkylene substituents may be the same or different, but will generally be the same owing to the methods of manufacture employed for such compounds. Useful alkylene groups tend to be restricted to the ethylene and propylene groups, but the propylene groups are not as good as the ethylene groups at preventing gassing, so that polyoxyethylene nitrogen containing compounds are preferred, especially the polyoxyethylene amines. It will be appreciated that it is possible for any given polyoxyalkylene moiety to contain a mix of lower alkylene groups, such as methyl, ethyl and propyl. Where this is the case, then we prefer the average alkylene length to be two, or close to two, carbon atoms.
The compounds of the invention are commonly available as surfactants, typically as provided in xe2x80x9cIndustrial Surfactants Electronic Handbookxe2x80x9d (published by Gower and edited by Michael and Irene Ash).
Regarding the nitrogen atom, it is particularly preferred that this is substituted by at least one polyoxyalkylene group and one alkyl group. The alkyl group may be substituted by one or more substituents, such as hydroxy groups and halogen atoms, but it is generally preferred that the alkyl group is unsubstituted. It is also preferred that the alkyl group should be straight chain and contain from 1 to 30 carbon atoms.
Compounds of the present invention may also contain more than one amine centre, in which case it is preferred that the individual amine groups are bridged by alkylene groups, preferably a short chain alkylene group such as a trimethylene group.
The chain length of the polyoxyalkylene group is not particularly important to the present invention, but we prefer that the chain length should be between 1 and 50, preferably with an average length of between 3 and 15 and especially around 10, an average length of 10 being the most preferred. Furthermore, compounds derived from coconut amines are preferred, and coconut alkyl groups contain between 6 and 18 (inclusive) carbon atoms. Thus, the most preferred compounds of the present invention are mono- and di-amines wherein the free alkyl group has between 6 and 18 carbon atoms, the side chains are polyoxyethylene substituents having an average of 10 oxyethylene units each and, where the compound is a diamine, then the link between the two amine centres is trimethylene. It should be noted that, while an average length of 10 is preferred, this is the overall average for the side chains of molecules in a given sample, so that the range of chain lengths may be from 6 to 18, but with an average chain length of 10. For example, Crodamet C20, a preferred compound, is a monoamine having two polyoxyethylene side chains, the number of oxyethylene units being 20 moles per mole of Crodamet C20.
Suitable formulae for preferred compounds of the present invention are as shown below. 
wherein R represents an optionally substituted alkyl group having from 1 to 30 carbon atoms, Rxe2x80x2 represents an alkyl group having from 2 to 10 carbon atoms, each m is the same or different and represents an integer from 1 to 4 inclusive, and n, x, y, and z are the same or different, and each represents an integer between 1 and 30. More preferred are compounds having the following formulae. It should be noted that, where compounds of the present invention are depicted by formulae, then the invention relates to either or both of the formulae. 
Particularly preferred compounds are those wherein R has an average of around 10 carbon atoms, Rxe2x80x2 has 3 or 4 carbon atoms, each m is 2 and n, x, y and z each averages about 10. The optional substituents are as noted above, but there are preferably no substituents.
The additives of the present invention may be added at any stage during the preparation of the electrochemical cell. There is no particularly preferred method of addition to the cells of the invention, provided that the additives are able to dissolve in the electrolyte.
One method of adding the additives to the cell is to coat a dilute aqueous solution of the additives on the inside of the can. The solvent is then allowed to dry out leaving a coated can. In fact, this method is suitable for testing new additives, but is not generally industrially practicable, although it is advantageous to provide the additive, initially at least, where it can act at the surface of the can.
The cells with which the additives of the present invention are typically used have a cathode made of manganese dioxide and acetylene black, the manganese dioxide being in finely divided form and mixed with acetylene black before mixing with the electrolyte, as is well known in the art. The additives of the invention may be mixed with the dry components of the cathode, or may be introduced together with the electrolyte. The mix is formed into the cathode and introduced into the cell where the pressure exerted on the cathode during insertion causes the electrolyte to escape slightly from the cathode so as to permeate the separator. This method of addition of the additive poses no problems, but requires sufficient additive to be able to be dispersed throughout the mix, typically in a proportion of about 0.01 to 2% w/w (additive/mix), preferably about 0.04 to 1%, and most preferably 0.1%.
We prefer to introduce the additive into the separator coating. To do this, it is generally necessary to add the additive and the gellant to the water before adding starch in order to provide the least complications with regard to uneven distribution. The coating can then be applied to the paper in a known fashion, and the resulting separator is then ready for use in an electrochemical cell. A suitable amount of the additive of the present invention to incorporate into an electrochemical cell will be readily apparent to a man skilled in the art. However, a suitable amount to add to the separator, for example, is, with respect to the dry coating weight of the coating, from 0.1 to 10%, more preferably from 0.5 to 5% and especially about 1.5%. It will be appreciated that this method of use of the additives of the invention is preferable to incorporation into the cathode mix, as it uses less additive.
Crosslinked starch molecules are a necessary part of the coating, as is a gum (or gelling agent). Advantageously, the additives of the present invention are used in conjunction with a coated paper separator, wherein the coating comprises a highly crosslinked starch and has an etherified cellulose derivative as a gelling agent.
The term xe2x80x9chighly crosslinkedxe2x80x9d is well known in the starch industry and, with respect to batteries, the preferred starches are corn, wheat and potato starches. Suitable examples of highly crosslinked corn starch include: Vulca 90 and Vulca 84 (Trademarks of National), Celex (Trademark of Nippon Starch Refining Company Limited) and the starches produced by Roquette, such as Lab 2211. Suitable examples of highly crosslinked potato starch include Vector R140 and Vector R120 (Trademarks of Roquette). A suitable example of a wheat starch is Lab 2214 (Roquette).
We prefer that the starch used in the coating is only a highly crosslinked starch, such as described above. If not, then it is desirable to keep the proportion of highly crosslinked starch as high as possible, preferably substantially over 50% of the dry weight of the coating mix, more preferably over 80% and ideally over 90%.
We also prefer to use etherified cellulose derivatives as the gellant, and suitable examples include: Tylose MH200K (Trademark of Hoechst), Tylose MH50, Culminal MHPC100 (Trademark of Aqualon) and Courtaulds DP 1209.
Etherified cellulose derivatives may be any that are suitable, by which is meant that the compound should swell and gel substantially immediately and remain stable in the presence of water.
Suitable examples of etherified celluloses include methyl cellulose, ethyl cellulose, hydroxymethyl cellulose, carboxymethyl cellulose (including salts, such as the sodium salt), hydroxyethyl cellulose, ethylhydroxyethyl cellulose, methylhydroxyethyl cellulose, 2-hydroxypropyl cellulose, methylhydroxypropyl cellulose and 2-hydroxypropylmethyl cellulose.
Particularly preferred combinations of etherified cellulose derivatives comprise or consist of Vulca 90 with Tylose MH200K, Tylose MH50 or Courtaulds DP 1209.
The nature of the paper to be used is not critical to the present invention, and may be any that is known in the art for use as a separator. Suitable simplex papers include Enso 80 (Trademark of Enso), Amatfors 57 and Sibille Dalle 64, while suitable duplex papers include PBDE 100 and PBDE 70 (NKK).
The compounds of the present invention (also referred to as xe2x80x9cadditivesxe2x80x9d herein) have been found to be most useful in electrochemical cells having acidic electrolytes. Use of certain additives of the invention, especially those with longer alkyl chains, such as those based on tallow, in cells having alkaline electrolytes can lead to increased gassing, thereby rendering them unsuitable for use in such cells.
Typical cells in which the compounds of the present invention can be used include primary and secondary zinc carbon cells, including those cells known as Leclanchexc3xa9 and zinc chloride cells. The electrolyte in such cells is typically as follows: Leclanchexc3xa9 electrolytexe2x80x945-20% zinc chloride, 30-40% ammonium chloride, remainder water; zinc chloride electrolytexe2x80x9415-35% zinc chloride, 0-10% ammonium chloride, the remainder water. Some other suitable cells for use in the present invention are described in Chapter 5 of the Handbook of Batteries and Fuel Cells (edited by David Linden, published by McGraw Hill).
The cells in which the additives of the present invention can be used may also be of any suitable configuration, such as round, square or flat and, in any of these configurations, it will be readily apparent to the man skilled in the art as to how to introduce the additives of the invention. Thus, in addition to the aspects described above, the present invention also provides a cell comprising a separator and/or a compound as described above, especially where the cell contains an acidic electrolyte when it comprises a compound of the invention.
In order to better assay cells using the additives of the invention, we have developed a further two new tests which we have termed the High Drain Continuous Test (HDCT) and the Low Drain Continuous Test (LDCT). The High Drain Continuous Test is intended to simulate abuse conditions, such as might be found in leaving a flashlight in the xe2x80x9conxe2x80x9d condition over a period of time, even after the battery had, to the user, gone xe2x80x9cflatxe2x80x9d. The Low Drain Continuous Test simulates the conditions experienced by a battery in, for example, a clock. HDCT results are measured in terms of the amount of leakage, whilst LDCT results are measured in terms of failure of the battery due to perforation or splitting of the can. Again, these tests are novel, and produce highly informative results in considerably less time than would otherwise be experienced in the conditions being simulated. Results are generally available in around 4 and 10 weeks respectively, although it will be appreciated that the amount of time required will depend on such factors as the cell which is to be tested and the extent to which it is desired to test the cell, for example.
These new tests (details of which are provided below) have enabled us to quickly and easily assay the effects of various constituents used in cell construction.
The Low Drain Continuous Text for an electrochemical Cell is characterised in that the can is sealed but left uncovered, a high resistance is secured between the poles of the cell so as to complete a circuit, and the cell is monitored as to its condition.
It will be understood that, in this test, monitoring the cell is intended to ascertain whether the call fails during testing. The typical lifetime of a D-size zinc carbon cell is up to about 10 weeks when the resistance is about 300xcexa9. Other resistances may be used as appropriate, although 300xcexa9 provides useful results. An appropriate resistance for a C-size cell is about 500xcexa9 while, for an AA-size cell it is about 810xcexa9. The omission of the bottom cover and the over tube is to expose the can to a surrounding atmosphere, thereby enhancing any failure that might occur, which is one reason why this test can be performed in 10 weeks, when it might take 2 years in a clock, for example.
The High Drain Continuous Test for an electrochemical cell involves the cell being preferably fitted with a bottom cover, a low resistance being secured between the top cover and a point on the can wall proximal to the top cover and, thereafter, sliding an overtube onto the can so as to cover substantially as much of the can as possible without dislodging the resistance, weighing the resulting assembly, storing the cell at ambient temperature, preferably 20xc2x0 C., weighing the cell at intervals during storage if desired, and determining the amount of electrolyte lost during storage by weighing to establish leakage. This last weighing may be effected by removing and weighing the over tube after storage or weighing the cell without the over tube but with the resistance, or both. Addition of the bottom cover during this test is particularly advantageous in preventing corrosion at the bottom of the can during the test.
A suitable resistance for this test for a D-size cell is 3.9xcexa9 and about 5xcexa9 for an AA-size cell, and the test is typically carried out for 4 weeks, testing at weekly intervals. The normal discharge life for a D cell is about 6 hours in this test until the cell becomes useless. Testing for 4 weeks, for example, establishes how the cell stands up to abuse conditions.
The present invention will now be illustrated with respect to the accompanying Examples wherein percentages are by weight, unless otherwise specified. The Test Examples are followed by certain Test Protocols appropriate to the Test Examples or which are not known in the art. Unless otherwise stated, the zinc cans used in the present examples typically comprise 0.4% lead and 0.03% manganese and have a wall thickness of 0.46xc2x10.03 mm. The mix for the cathode typically comprises 52% manganese dioxide, 0.4% zinc oxide, 6% acetylene black and 41.6% zinc chloride solution (26.5% zinc chloride w/v). Otherwise, cells are generally manufactured in accordance with EP-A-303737.